1. Field of the Invention
The present invention relates to a pattern measuring apparatus and a pattern measuring method which use a charged particle beam, and relates particularly to a pattern measuring apparatus and a pattern measuring method which can distinguish lines and spaces from one another in a line and space pattern in which the pattern width and spacing are equal.
2. Description of the Related Art
Measurement using a scanning electron microscope is performed as a method of measuring a line width of a pattern. The scanning electron microscope irradiates and scans a region within an electron beam scanning area with incident electrons, obtains secondary electrons emitted from a sample by using a scintillator, converts the amount of the obtained electrons into luminance and display an image on a display device.
The management of the characteristics of a semiconductor device by using such a scanning electron microscope generally includes a work of checking whether a pattern is formed to have the line width within a design standard. The management of the line width of the pattern is performed in the following procedures. A predetermined area of the pattern formed on a photomask is displayed on a display. Thereafter, an electron beam is aimed at a measurement point within the displayed area, and the measurement point is irradiated with the electron beam. Then, a waveform of luminance distribution is obtained based on secondary electrons reflected from the measurement point. Subsequently, the width of a portion where the waveform of the luminance distribution is at a high level is determined as a line width. It is determined whether the line width is within an allowable margin of error. If the line width is within the allowable margin of error, the processing proceeds to the next step. If the line width is not within the allowable margin of error, the processing returns to a step of pattern formation processing.
As described above, the measurement of the line width of the pattern is important in a manufacturing process of the semiconductor device, and various methods are proposed to accurately measure the line width.
Generally, a position at which the inclination of the luminance corresponding to the amount of secondary electrons is the largest is set as an edge position of the pattern. Meanwhile, Japanese Laid-open Patent Publication No. 05-296754 discloses such an edge detection method that a position at which a secondary electron signal takes the minimum value is considered as the edge position.
Moreover, Japanese Laid-open Patent Publication No. 2005-195361 discloses a method in which luminance information of a line and space pattern is subjected to primary differentiation, and average line width and space width are calculated from an auto-correlation value between an original image and an image shifted in an x direction.
Meanwhile, if a line pattern and a space pattern have almost the same width, it is sometimes difficult to judge whether a region between two edges is the line pattern or the space pattern even if the edges are detected. For example, as shown in FIG. 1A, since the tone of a line pattern 103a is generally higher than the tone of a space pattern 102a, the pattern 103a with a high tone between edges 101 can be judged as a line pattern.
However, depending on the material and film thickness of the pattern, there may be no difference in tone between the line pattern 103b and the space pattern 102b as shown in FIG. 1B, or the tones may be reversed in the line pattern 103b and the space pattern 102c as shown in FIG. 1C. In such cases, the portion between the edges 101 may be erroneously judged as being a line pattern or a space pattern.
Japanese Laid-open Patent Publication 2007-292732 describes a technique of identifying whether a portion is a line pattern or a space pattern in a line and space pattern in which a line width and a space width are formed to be almost equal.
In the technique of Japanese Laid-open Patent Publication 2007-292732, detection can be made on whether an edge of a line pattern is rising or falling on the basis of a secondary differentiation profile of a line profile, in the case where a line pattern and a space pattern have almost the same width.
Specifically, the line profile is calculated from a SEM image of a pattern, and processing of obtaining the secondary differentiation profile from the line profile is performed, and determination of whether the edge is rising or falling is made based on the peak position and peak value of the secondary differentiation profile near the edge of the pattern.
However, when the peak value of the secondary differentiation profile is not clearly detected, it is difficult to accurately judge whether the edge is rising or falling.